Many British couples run out of steam after having sex because they fail to drink enough water, according to a study.

Doctors say the finding could explain why many people develop headaches or become lethargic after intercourse.

They have warned that couples who work up a sweat in bed risk dehydration unless they replenish their fluids.

The findings contrast with results from France where people drink five times as much water and have more sex than their British counterparts.

The study found that just two out of five people drink water after sex.

Sweating, flushing and panting are all signs that the heart rate is faster

Dr John Leiper, Aberdeen University

This is despite the fact that half an hour of making love can be as strenuous as a three-mile run or a good workout in the gym.

In contrast, the survey, which was carried out on behalf of mineral water company Highland Spring, shows that nine out of 10 people drink water after running or going to the gym.

Dr John Leiper, an expert in fluid balance and hydration at Aberdeen University, stressed the importance of drinking water after sex.

He said many people were unaware that intercourse can be as dehydrating as other forms of exercise.

"Sweating, flushing and panting are all signs that the heart rate is faster, meaning that the body is reacting in the same way to sex as it does to other cardiovascular exercises.

"As we go about our daily activities we continually lose water, even when we are not visibly sweating, so when we take part in exercise or sexual activity this water loss increases and we need to replace it to prevent dehydration."